In recent years, a rapid increase in recording density has been achieved in magnetic recording devices such as HDDs. An information recording medium adapted to be mounted in the HDD is a magnetic disk and is manufactured by forming a magnetic layer and so on on a substrate such as an aluminum-based alloy substrate or a glass substrate. In the HDD, a magnetic head records an information signal on the magnetic layer as a magnetized pattern or reproduces it while flying over the magnetic disk rotating at a high speed.
Recently, attention has been paid particularly to a glass substrate as a magnetic disk substrate suitable for increasing the recording density. Since the glass substrate can achieve a smooth surface, it is suitable for a magnetic head with low flying height and is a substrate suitable for improving the S/N ratio of a recording signal and increasing the recording density. That is, the glass substrate can be said to be a substrate excellent in adaptation to the low flying height of the magnetic head. However, the glass substrate is a high-strength and high-rigidity material on the one hand but is a fragile material on the other. Therefore, various strengthening methods have been attempted. As a glass strengthening method normally used, there is a crystallization method or a chemical strengthening method. As a chemical strengthening method of a glass substrate for an information recording medium such as a magnetic disk, there is known, for example, a technique of Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2000-203888) or a technique of Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2001-72444).